A traditional injection molding system melts a material, such as a plastic, primarily by shear heat that is dynamically generated by rotation of an extrusion screw. Dynamically generated shear heat in the traditional injection molding system is dependent on the use of petroleum-based plastic resins of a high level of purity and consistency. FIG. 1 is a schematic diagram for a traditional injection molding system 100. An injection zone 112 is located in front of an extrusion screw 102 to hold a molten material prior to injection. A check ring 104, or a non-return valve, is used to allow a forward melt flow during a recovery extrusion stage that is between shots and to prevent the molten material from back flow to the extrusion screw 102. The back flow may occur when an injection pressure is applied to the melt. The material may be molten by using mostly shear heat. For example, the molten state may be created by about 75% shear heat and about 25% conduction heat generated from band heaters 114.
The traditional extrusion screw 102 is designed with a large pitch 132 to promote shear heat generation and mix hot and cold plastic. As shown in FIG. 1, a root diameter 134 of the screw 102 is narrower near a hopper 106 which feeds raw material through an inlet of a barrel 110. Along the length of the extrusion screw toward the nozzle 108, the root diameter increases to create a compression zone to promote shear heat generation. A flight height 136 of the screw 102 decreases toward the nozzle 108, which reduces the space between the screw 102 and the barrel 110.
During a recovery extrusion stage, the molten material is transported along the length of the screw 102 into the injection zone 112 in the barrel 110 by rotating the extrusion screw using a motor 150. The injection zone 112 is between a nozzle 108 and the check ring 104 at the end of the extrusion screw 102. The molten material is trapped in the injection zone by the cold slug, which seals the nozzle 108 after the injection cycle and prevents the plastic from flowing into a mold 140 through a gate 146 and runners 142 during the recovery extrusion stage.
During an injection cycle, the screw 102 is driven forward without rotation under a very high injection pressure by cylinder 138. The screw 102 and check ring 104 can function together as a plunger to inject the molten material into the mold. The recovery extrusion stage may take only 10-25% of the entire molding time such that the shear heat may also be lost when the extrusion screw does not rotate except during the recovery extrusion stage.
The traditional injection molding system 100 relies on the formation of a cold slug in the nozzle 108 between each shot. The cold slug of plastic causes one of the greatest inefficiencies for the traditional injection molding system 100. The cold slug requires a very high pressure to be dislodged from the nozzle 108 to allow a molten material to flow into a mold cavity. The high injection pressure is required to push the molten material into the mold cavity through the runners 142. It is common to require an injection pressure between 20,000 and 30,000 psi in order to obtain a pressure of 500 psi to 1,500 psi in the mold cavity. Due to the high injection pressure, the traditional injection molding system 100 requires a thick wall of the barrel 110, which reduces the heat conduction to the material from the band heaters 114 that surround the barrel 110.
The traditional injection molding system 100 may use either a hydraulic system or an electric motor 128 for powering a clamp system 120, which may include stationary platens 122A-B, a moveable platen 124, and tie rods 126. A clamping cylinder 130 applies sufficient pressure to hold the mold closed during injection. The traditional injection molding system requires large and costly power sources for both the injection system 118 and the clamp system 120. These power sources must be supported by a massive machine structure, which increases facility infrastructure costs including electrical supply, thick concrete footings or floors and oversized HVAC systems that are expensive to procure, operate and maintain.
The shear heat generated by the traditional injection molding system limits its capability to mold certain materials, such as bio-based plastics. Bio-based plastics are degraded by the pressures applied in the traditional injection molding system, reacting adversely to the pressure the machine generates for creating shear heat in process of injection molding petroleum-based plastics. A recently developed injection molding system disclosed in U.S. Pat. No. 8,163,208, entitled “Injection Molding Method and Apparatus” by R. Fitzpatrick, uses static heat conduction to melt plastic, rather than shear heat. The disclosed system can mold bio-based plastics into small parts. Specifically, the disclosed system includes a plunger that is positioned within a tubular screw and runs through the center of the tubular screw. Generally, moving the entire screw forward during the injection cycle would require a large injection cylinder. In the disclosed system, the entire screw of a larger diameter does not move. Only the plunger is advanced, which requires a much smaller injection cylinder to apply the force on the plunger. The disclosed system recovers and transports the molten material in front of the plunger between each shot or injection cycle, and injects the molten material into a mold by the plunger. The part size is determined by the area of the plunger multiplied by the length of plunger stroke as that defines the volume during injection, but that part size is limited to the small displacement volume of the plunger, typically about 3-5 grams of plastic, which is a small shot size. It is desirable to mold parts with unlimited shot sizes.
Also, the traditional injection molding system 100 requires a manual purging operation by experienced operators at start-up. For example, an operator may first turn on the barrel heaters 114 and wait until the screw 102 embedded in plastic or resin is loosened to allow the screw motor 150 to be turned on. A purging process is required for generating initial shear heat. The purging process begins when the operator rotates the screw 102 to move the resin forward, and the screw 102 is driven backward into its injection position. Then, the operator activates the injection force to drive the screw 102 forward, allowing the resin to exit the nozzle 108 onto the machine bed. The cycling process is repeated to generate initial shear heat until the resin exits from the nozzle 108, which suggests that the material may be hot enough such that the operator may start molding. The manual operation is highly subjective and requires skilled operators to start machines and adjust molding processes. The subsequent molding operations must be consistent without interruptions to satisfy shear heat generation requirements.
Documents that may be related to the present disclosure in that they include various injection molding systems include U.S. Pat. No. 7,906,048, U.S. Pat. No. 7,172,333, U.S. Pat. No. 2,734,226, U.S. Pat. No. 4,154,536, U.S. Pat. No. 6,059,556, and U.S. Pat. No. 7,291,297. These proposals, however, may be improved.